There are many potential applications for optical amplifiers and lasers that are efficient, low mass, and compact in size, yet can be scaled to high average powers while producing a high quality, near diffraction unlimited, beam. For example, laser beams are used extensively in industry for materials processing, cutting, and drilling applications and in medical surgical procedures in which very narrowly focused, high intensity beams produce sharper, cleaner cuts. A TEM00 beam is one type of beam in which the light energy is spatially coherent (same phase across the thickness or cross-section of the beam) and is the lowest spatial mode of a laser. (Spatial mode in context of spatial coherency refers to the degree to which the laser is spatially coherent and should not be confused with modes of light transmission in a waveguide, which are also discussed herein.) A TEM00 beam can be focused down to the smallest size—much more so than higher modes, thus concentrating the light energy in the beam to a high intensity. A TEM00 beam can also be propagated for the longer distances with reduced beam spreading out in size. For many applications, therefore, it is desirable to pack as much energy as possible into TEM00 beams. For example, for cutting materials, packing more energy into a TEM00 beam means more power that can be focused to a very small spot to cut better, sharper, and cleaner than, a higher mode, e.g., TEM01 or TEM10, in which the light energy has even less spatial coherency of the light energy in the beam.
Laser light can also be used to detect range (distance away), speed and direction of travel, and even shapes or images of objects, such as targets, as described, for example, in U.S. Pat. No. 5,835,199. Range can be measured by transmitting (launching) short pulses (bursts) of laser light toward the object, e.g., target, from the range detector. When each pulse of light energy reaches the target, the light energy is reflected by the target. Unless the target is a high-quality mirror, the reflection scatters the light energy in many directions. However, a small portion of the reflected light energy will return to the range detector, where instrumentation can detect the reflection as well as the time it took for the pulse of light energy to travel from the range detector to the target and back, i.e., time of flight. Since the velocity (speed) of light is known, the time of flight can be used with the speed of light to calculate the range (distance) of the target from the range detector. If the target is moving in relation to the range detector, the reflected light energy will have a frequency change, known as the Doppler shift, which can also be detected in the small portion of light energy reflected back to the range detector. Such Doppler frequency shift can be used to determine the direction of travel and speed of the target in relation to the range detector. Such range and velocity (distance, speed, and direction) detection can only occur however, if the previously described small portion of the original light energy that gets reflected and returns to the range finder is still strong enough to be detected in the midst of all other light energy of similar wavelengths in the atmosphere (background noise), which also reaches the range detector. Of course, some of the light energy is also absorbed and dispersed by the atmosphere. Therefore, the more light energy there is in the pulse of light that reaches the target, the more likely it will be that enough light energy will be reflected by the target back to the range detector to be detectable and distinguishable against the background noise. A TEM00 beam will propagate farther without spreading than any other mode (degree of spatial coherency), thus will keep the light energy concentrated in a smaller space than other modes. Consequently, a TEM00 beam pulse will be able to deliver more of its energy to the target back to the range detector than other modes, which will enhance the likelihood that the return reflection of light energy from the target will be of sufficient amplitude (strong enough) to be detectable by the range detector.
The maximum range that can be measured, therefore, is determined, at least in part, by the energy contained within the pulse of laser light. As explained above, enough light energy has to be able to travel the full distance from the range detector to the target (surviving absorption, scattering, and other attenuation by the atmosphere), be reflected (surviving absorption and reflective scattering in other directions by the target), and return to the range detector (surviving still more absorption, scattering, and other attenuation by the atmosphere) and still be of larger amplitude than the background noise. The longer the pulse, i.e., the longer the laser is left turned “on”, the more light energy there will be in the pulse. Therefore, to detect targets at the longest range (distance away), the shorter pulses with near diffraction limited, spatial coherency and near transform limited spectral content are often desired. However, if there are several targets or objects close to each other, the long pulse will not allow the range detector to distinguish between light energy reflected from the several targets or objects, respectively. For example, if the transmitted light energy in the pulse extends for 20 meters in length, the reflection will occur for the full 20 meters of the pulse and thereby produce a 20-meter reflection, which might be fine for one object or target. However, if one object or target is positioned only ten (10) meters apart from another object or target, the range detector would not be able to detect that there are actually two objects or targets spaced ten (10) meters apart from each other, instead of just one target or object. Such range discrimination, i.e., the minimum distance separating two reflective surfaces that can be detected separately, is even more critical in laser imaging applications in which the range detector must be able to discriminate between different reflecting surfaces of the same object or target in order to determine its shape. Such imaging along with range detection may be used, for example, to distinguish between an enemy tank and an adjacent house or to determine if an airplane has the shape of a commercial airliner or a military bomber.
Consequently, as a general rule, temporally short light energy pulses, i.e., low pulse widths, have better range or even shape discrimination capabilities, but the maximum range is often poor because of the low energy content that is technically practical in previous, state-of-the-art systems. Conversely, the wider the pulse, the higher the energy can be, and the longer the range, but, at the same time, the worse the range or shape discrimination will be.
For the example applications described above, as well as many more, it would be very beneficial to be able to provide higher power TEM00 beams with compact equipment that is not subject to typical adverse, non-linear and thermal effects, which limit or degrade performance of current state-of-the-art high power laser resonators and high power optical amplifier devices. Such non-linear and thermal effects include thermal self-focusing and self-phase modulation, stress birefringence, stimulated Rayleigh, Raman, and Brillouin scattering, intermodel dispersion, and the like. Current state-of-the-art approaches to providing high power beams include, for example, using high-efficiency laser diodes to pump crystalline laser materials in the form of rods of slabs. Since the power from a single mode laser diode is currently limited to about one watt in state-of-the-art devices, large arrays of diode emitters are used to scale to higher powers. The poor spatial mode quality of diode arrays requires novel pump geometries to achieve sufficient overlap of pump and laser beams. A variety of laser diode coupling methods have been demonstrated, including lens ducts for end pumping [Camille Bibeau et al., “CW and Q-switched performance of a diode end pumped Yb:YAG laser,” Adv. Solid State Lasers, vol. 10, 296 (1997)], side coupling into a diffuse pump chamber [S. Fiyikawa et al., “High-power high-efficient diode-side-pumped Nd:YAG laser,” Adv. Solid State Lasers, Vol. 10, 296 (1997)], fiber-coupled diode arrays for longitudinal pumping, and direct transverse pumping in slab lasers [A. McInnes et al., “Thermal effects in a Coplanar-Pumped Folded-Zigzag Slab Laser,” IEEE J. Quantum Electron Vol. 32, 1243 (1996) and references therein]. At high powers, it is difficult to achieve both excellent pump/mode matching with high pump absorption and diffraction-limited beam quality. Longitudinal pumping can result in excellent mode matching, but it is limited in power due to the thermal stress fracture limit, i.e., the medium will crack when it gets too hot [S. Tidwell et al., “Scaling CW Diode-End-Pumped Nd:YAG Lasers to high Average Powers,” IEEE J. Quantum Electron, vol. 28, 997 (1992)]. One common problem in all these bulk laser geometries is thermal management—both in the form of heat extraction and dissipation and optical distortion due to thermal gradients. The heat build-up results from absorption of the high pump energy in a small volume of laser material, and active cooling in the form of bulky heat exchangers or refrigeration systems is usually required to remove the heat. Such active cooling adds severely to the size, weight, and power requirements of the system. Thermal gradients in the laser materials are manifested in the forms of undesirable thermal lensing or self-focusing, due to thermally-induced birefringence, which alters polarization of the light. See, for example, David Brown, “Nonlinear Thermal Distortions in YAG Rod Amplifiers”, IEEE j. Quantum Electron, vol. 34, 2383 (1998). Considerable research has been devoted to compensation schemes for these adverse thermal effects. These problems are significant, because there is typically power dependent birefringence and bi-focusing. See, James Sherman, “Thermal compensation of a CW-pumped Nd:YAG laser”, Appl. Opt., vol. 37, 7789(1998). One technique that has been tried to alleviate this effect is to use extremely thin laser media (“thin disks”) such that thermal gradient is reduced and one-dimensional. See U.Branch et al., “Multiwatt diode-pumped Yb:YAG thin disk laser continuously tunable between 1018 and 1053 nm”, Opt. Lett., vol. 20, 713 (1995). However, operation in quasi-three-level laser material (Yb, Er, Tm, Ho) severely exacerbates the thermal problem, since it requires much higher pumping to reach threshold and/or refrigerated cooling to depopulate the thermal laser level. Consequently, there has not been any real solution to the thermal problems when scaling bulk laser materials to high power levels.
Optical fiber lasers and amplifiers overcome some of the thermal problems of bulk laser crystal materials by greatly increasing the length of the gain medium and providing mode confinement, i.e., limiting the size of the fiber core diameter so that it can only propagate the lowest order eigenmode, (so-called “single mode fibers“—”mode” in this context not to be confused with spatial coherency modes discussed above and below). There are several benefits to this approach, including: (i) the long interaction length between the pump light and the laser beam lead to high gain and efficient operation, even in 3-level lasers in which the terminal laser level is thermally populated; (ii) Heat is distributed over a longer length of laser medium with a larger surface area, so the heat can be dissipated with passive conductive cooling to the atmosphere or to a heat sink; (iii) Operation can be restricted to a single transverse mode, which preserves a TEM00 spatial coherence for the beam focusability and beam propagation with minimal beam spreading advantages as described above; (iv) The flexible nature of the optical fibers allows compact and novel optical designs; (v) The optical fibers can be directly coupled to other passive or active waveguides for modular functionality; and (vi) Fabrication is suited to large-scale production, which reduces costs. However, power scaling, i.e., scaling up to higher power levels, in such single-mode optical fibers is restricted by inability to make efficient coupling of pump light energy into the optical fiber and by the minute, single-mode core, (less than 10 μm diameter), which can only handle so much light energy without overheating and resulting in catastrophic facet (coupling surface) failure. The very small diameter, single-mode core size of a single-mode fiber has a very small numerical aperture (optical opening) through which light can be introduced into the fiber core, so high intensity, narrowly confined or focused pump light sources must be used.
This limitation of fiber lasers and amplifiers has been partly overcome by use of a double-clad fiber structure in which the small-diameter, single-mode core is surrounded by an inner cladding region, which, in turn, is surrounded by an outer cladding region. The inner cladding region has a larger numerical aperture than the core, thus can accept more pump light energy in more modes. Therefore, the pump light is optically confined to both the core and inner cladding regions together, while the optical beam (preferably a TEM00 beam) is confined to the core alone. However, drawbacks of such double-clad fiber designs include: (i) The pump light energy, while introduced into, and confined by, the core and inner cladding together, is absorbed only in the core region so that the effective absorption coefficient is reduced by approximately the ratio of the core area to the inner cladding area; (ii) The inner cladding size is still very small, even though larger than the core, so that coupling of a laser diode array into the inner cladding region is still quite difficult and not very efficient; and (iii) The outer cladding region must be made with a much lower index of refraction than the inner cladding for optical confinement of the pump light to the inner region, and such lower index of refraction materials are often polymers (plastic), which are much more susceptible to damage than glass, especially from heat.
Essentially, the single-mode core diameter of optical fibers is so small (less than 10 μm, which is equivalent to 7.8×10−7 cm2 in cross-sectional area) that a 10 μJ (micro joule) pulse of light has a fluence (energy per unit area) greater than 13 J/cm2 (joules per square centimeter), which is close to the damage threshold of the fiber. Larger core diameter can handle more energy, of course, so that a 10 μJ pulse of light would not be so close to the damage threshold, but larger core diameters result in undesirable eigenmode mixing and resulting loss of polarization, spatial coherence, and temporal coherence. Polarization is required for many beam input and output systems as well as beam splitting and analysis functions, and TEM00 spatial coherence has the focusing and non-spreading spreading propagation benefits described above. Therefore, loss of polarization and spatial coherence are significant beam degradations that should be avoided. Some complex-design, large-area, multi-mode fibers have been reported with reduced mode-mixing and pulse energies up to 500 μJ with M2<1.2, where M2 is a measure of divergence relative to diffraction limit and M2=1 is diffraction limited, have been reported [see, e.g., H. Offerhaus et al., “High-energy single-transverse mode Q-switched fiber laser based on multimode large-mode-area erbium-doped fiber”, Opt. Lett., vol. 23 (1998)], but no truly single mode (LP01) fiber design has been able to break the 1 mJ (1,000 μJ) barrier, while maintaining spectral and spatial coherence with short temporal pulse widths.
In some applications, production and amplification of high-power, high quality laser and other light beams is only part of the problem. Transporting such high-power, high-quality beams to points of application, such as the industrial cutting and materials processing, medical, laser radar ranging, imaging, and tracking applications mentioned above, can also present heretofore unsolved problems. For example, in laser radar (ladar) system described in U.S. Pat. No. 5,835,199, a high-power laser beam is produced for launching from airplanes or other platforms for ranging, imaging, and tracking objects or targets as much as twenty miles away or more. In an airplane, the most effective launch point for such high-power beams may be in the nose cone of the airplane. However, the nose cone is usually small, and there are also many electronic and other kinds of equipment that also have to fit there. Consequently, it is often not possible to place the high power beam production and amplifying equipment described in U.S. Pat. No. 5,835,199 at the most effective launch location in the airplane. It would be very beneficial to have some way of transporting only the high power beam from some other location in the airplane to a launch point in the nose cone without degrading beam power, quality, polarization, and the like.
Similar beam transport capabilities would also be beneficial in industrial, medical, imaging, directed energy, and other applications of high power laser and other light beams, where space is limited or where it would just be more convenient to place a high powered, high quality beam without all the associated beam production and/or amplification equipment.
Yet, transport of high power, high quality laser beams without degradation of beam power, quality, temporal and spatial coherence, polarization, and the like presents serious problems with many of the same kinds of obstacles as described above for the beam production and amplification. For example, single mode waveguides, such as single mode optical fibers, can maintain beam quality, but are very limited in power transport capabilities. It is not uncommon for industrial medical, and even imaging applications to require continuous wave (cw) output power of 100 watts or more, while even higher power laser applications, such as Q-switched or pulsed lasers, may have output power in the megawatt range, such as 10 megawatts or greater. Single-mode fibers and waveguides are simply unable to handle that kind of optical power or light energy.
Multi-mode fibers and other waveguides can transport more power, but they do not maintain spatial coherence, polarization, and the like, because of multi-mode interference and other reasons mentioned above. Free-space light transport has its own problems, not the least of which is that the light paths have to be unobstructed and alignment and stability problems in non-laboratory environments are extremely difficult to overcome and are often insurmountable.
Techniques have been previously developed to actively compensate for finite length circular fiber spatial mode deficiencies, potentially including SBS phase conjugation, but these techniques are limited in scope to narrow spectral linewidth lasers to match the SBS gain bandwidth, enough optical power to provide the nonlinear drive field required and wavefronts that are not fully randomized. Furthermore, and as previously mentioned, it may be desirable in many waveguide applications to maintain polarization. In circular fibers with a uniform index-of-refraction in both the core and cladding, polarization may not be maintained. To preserve polarization, special polarization-maintaining fiber designs may be required which essentially create an asymmetric index difference in orthogonal directions. If this index profile is disturbed, potentially as a result of high power operation, the polarization integrity may drift or be lost.
Previous attempts may have been made to incorporate traditional self-imaging techniques for low power waveguide systems to potentially preserve beam quality. The concept of self-imaging, generally, may have been derived in accordance with traditionally known physical observations. According to one such observation, it has been demonstrated generally that if a plane wave illuminated a planar phase or amplitude grating, such as a Ronchi grating, that the grating would periodically spatially re-image without the use of lenses. The imaging period along the propagation axis is generally referred to as the Talbot distance:DT=2 n a2/λ  (1)where n is the index of refraction, a is the waveguide width, and λ is the wavelength of light in a vacuum. However, such application of Talbot self-imaging may not have been heretofore properly applied for high-power waveguide systems.
U.S. Pat. Nos. 3,832,029 and 4,087,159, hereby incorporated by reference, may provide self-imaging techniques for low power, image-forming waveguide systems and particular configurations of waveguides for self-imaging. However, both systems may suffer from draw backs related to high power operations. The potential incorporation of such waveguide systems into high-power applications such as laser applications, and directed energy systems, object imaging systems, object positioning and tracking systems, detection systems, fiber optics, machine fabrication, and medical systems, generally, may result in potential optical damage to the waveguide and nonlinear optical effects, as previously described, as such previous systems appear not to accommodate for high power operations. Additionally, a potential long felt but unmet need may have existed in relevant fields regarding the resolution of beam quality and high power application aspects for waveguide systems, wherein the efforts described in U.S. Pat. Nos. 3,832,029 and 4,087,159, and potentially other previous attempts, may have failed to even address such operations. Therefore, such waveguide systems may actually teach away from the incorporation of self-imaging techniques and particular configurations of waveguides in high-power waveguide and beam transport techniques.
Beam quality issues may arise, for example, related to mode mixing as previously described, or with regard to “bend, buckle and twist” of the waveguide and potentially resulting modification of at least spacial coherence, wherein, for example, a twist of the waveguide may optically result beam formation potentially equivalent to a negative lens, and a bend in the waveguide may result in beam formation potentially equivalent to a positive lens. Such applications of waveguide technology have not been adequately addressed in the past attempts previously described or in other previous beam transport technologies.